Functional consequences of the common diseases of aging are of particular importance in geriatric medicine. Breast cancer is the most common neoplasm in women, with an increase in incidence with age. The diagnosis and treatment of breast cancer results in decreased functioning in physical and psychosocial domains, which have a major effect on an older woman's daily life. The existing studies of quality of life outcomes after breast cancer have been hampered by several limitations, such as failure to consider age-related differences; lack of pre-illness measures of functional status; absence of appropriate comparison groups (disease-free women); small sample sizes; and lack of standardization in instruments selected to measure quality of life. The proposed study will prospectively examine the age-related impact of breast cancer on physical and psychosocial function among approximately 3,520 cases of breast cancer. The study will take place in two large on-going longitudinal studies: the Nurses' Health Study (NHS) and the Nurses' Health Study II (NHS-II). The NHS is a prospective cohort of 121,700 community dwelling women aged 50-76 in 1996 and the NHS-II is a similarly designed prospective cohort of 116,686 women aged 35-50 in 1996. We propose to undertake a comprehensive assessment of function on two levels -- generic quality of life and cancer-specific quality of life. The particular strengths of our proposed study include pre-cancer levels of physical and psychosocial function; ability to compare functional outcomes among older breast cancer patients to healthy controls as well as younger patients; prospective study design; large sample size; high response rate; extensive information on potential confounding factors and effect modifiers (such as disease severity and treatment, co-morbid conditions, and social networks); and the use of standardized and well-validated instruments to assess functional status and quality of life.